Well, it used to flag new posts of mine if I posted say from home and then went to work and checked from there. It always did that. Now though it seems to highlight them if I just close the browser and reopen it.
The problem of it losing all the new post flags after viewing one new post is new though. It's kind of annoying since I don't always remember what threads/forums were highlighted. Like I had to specifically check this post to know that you had replied to it.

Just a minor annoyance.